It is common for implantable cardiac stimulators, such as pacemakers and defibrillators, to be implemented in the form of various circuits under the control of a microprocessor. The functions these circuits perform include pulse generation, cardiac electrical signal sensing, performance of various diagnostic and physiological measurements and communication between the implanted stimulator and an external communicating device, commonly called a programmer. One of the problems with cardiac stimulator implementations is that they require large current consumptions and, therefore, high battery drains, which tend to result in short product lives, large product sizes or a combination of these two disadvantageous conditions.
One early current-saving improvement to microprocessor-controlled pacemakers was the implemention of "wakeup" control. P. L. Gordon et al., in U.S. Pat. No. 4,404,972, entitled "Implantable Device with Microprocessor Control", issued on Sep. 20, 1983, disclose a pacemaker in which its controlling microprocessor operates only in response to various external physiological events and internal timer events. These physiological events and timer events act upon internal logic circuits which, in turn, generate wakeup signals acting upon the microprocessor. The microprocessor enables and disables these internal logic circuits to determine which wakeups may activate the microprocessor. The microprocessor responds to a wakeup signal from an internal logic circuit by becoming active and performing a wakeup routine. At the end of this routine, the microprocessor determines which set of wakeup events are active and which are inactive. Then the microprocessor sends control signals to activate and deactivate the appropriate internal logic components. In the final step of the wakeup routine, the microprocessor deactivates itself.
Battery operated microprocessors use complementary metal-oxide-silicon (CMOS) circuits to reduce power consumption. However, to take full advantage of CMOS circuits, the circuits must not be clocked when their usage is not required. CMOS circuits provide for very low standby power consumption when they are not being clocked. Thus, it is known that the useful life of a battery in a microprocessor-controlled implantable device may be extended by turning off power to nonessential circuits at certain time periods during which such circuits are not required.
Present day pacemakers begin pacing immediately upon manufacture and continue to pace at an amplitude and rate which are deemed to be safe for nearly all cardiac patients. Pacemakers are not turned off between the time of their manufacture and the time of their implantation because of the risk of implanting them while they are in a turned-off condition. A pacemaker may, thus, sit on a shelf for months while it is continuously pacing and wasting power.
When using a battery-powered control system, such as that within a pacemaker, it is desirable to extend the life of the power source without jeopardizing the functioning of the control system.
It is desirable to be able to turn off an implantable stimulator after it is manufactured while assuring that it will be activated into a known state before it is implanted.
It is, therefore, a primary object of the present invention to provide a method and apparatus for deactivating a stimulator into a quiescent state, during which only essential functions of communication and memory maintenance are enabled, and for reactivating the stimulator into a known active state.
It is also an object of the present invention to provide, in an implantable device powered by a limited capacity electrical power source, a lower level of power consumption to thereby extend the life of the power source and expand the shelf life of the device.
It is an additional object of the present invention to lower the level of power consumption and extend the life of the power source without altering stimulator functionality or integrity.
It is another object of the present invention to provide for a communication link between an implantable stimulator and an external device which always remains operable even though the stimulator may be placed in a quiescent state.
It is further object of the present invention to maintain any information stored in volatile memory in an implantable stimulator, even though the stimulator may be placed in a quiescent state.
It is yet another object of the present invention to provide a method and apparatus which distinguish a cardiac stimulation system reset condition caused by the quiescent state from a system reset condition caused by an error.
It is a still further object of the present invention to provide a method and apparatus for placing a cardiac stimulator in a quiescent state, which method and apparatus are fool-proof and prevent a stimulator from being implanted in its quiescent state.
Further objects and advantages of the invention will become apparent as the following description proceeds.